JP2017108283A - Satellite receiver and reception method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple-structured and light-weight satellite receiver.SOLUTION: A reception antenna 52 captures a radio signal arriving from a communication satellite, and transmits the radio signal to a reception RF unit 60. The reception RF unit 60 down-converts the radio signal into a reception IF signal, and inputs the reception IF signal to a reception IF unit 70. A local oscillator 30 generates a local frequency signal. The reception IF unit 70 mixes the local frequency signal with the reception IF signal, and inputs the mixed signal into a demodulation unit 80. The demodulation unit 80 orthogonally demodulates the reception IF signal to generate baseband IQ complex signals orthogonal to each other, and inputs the signals into a processing circuit 90. The processing circuit 90 reproduces transmission data included in the received radio signal, from the IQ complex signals, and outputs the reception signal.SELECTED DRAWING: Figure 2

Description

  Embodiments described herein relate generally to a satellite receiver and a reception method applicable to, for example, VSAT (Very Small Aperture Terminal).

  VSAT is known as a device having a relatively small antenna aperture among devices for communicating with geostationary satellites. Car-mounted hardware is also available, and its use at the disaster site has begun utilizing its mobility. Often used in conjunction with mobile communications infrastructure.

JP 2002-271224 A

  In this type of system as well, in recent years, a multi-channel / wide-band VSAT has been called wideband and multi-channel. Although the structure of the receiving system tends to be inevitably enlarged, there is a demand for a small and lightweight satellite receiving apparatus that can be suppressed by some device.

  An object of the present invention is to provide a satellite receiver and a receiving method thereof that are simple in structure and light in weight.

  In the embodiment, the satellite receiver includes a high frequency unit, a local oscillator, an orthogonal demodulation unit, and a signal processing unit. The high frequency unit converts a radio signal received from an artificial satellite including a plurality of transponders into an intermediate frequency signal. The local oscillator outputs a local frequency signal having a frequency included in a guard band between the cover bands of each of the plurality of transponders. The orthogonal demodulator performs orthogonal demodulation on the intermediate frequency signal using the local frequency signal output from the local oscillator, and generates baseband IQ complex signals orthogonal to each other. The signal processing unit reproduces transmission data included in the radio signal from the IQ complex signal.

FIG. 1 is a diagram illustrating an example of a satellite communication system using VSAT. FIG. 2 is a functional block diagram illustrating an example of a VSAT station according to the embodiment. FIG. 3 is a functional block diagram illustrating an example of the reception IF unit 70 and the demodulation unit 80. FIG. 4 is a diagram illustrating an example of the relationship between the cover bands T1, T2, and T3 of each transponder and the frequency Lo of the local frequency signal. FIG. 5 is a diagram for explaining direct conversion based on the frequency setting of FIG. FIG. 6 is a diagram schematically showing the relationship between the cover bands T1, T2, T3 and the guard band after down-conversion. FIG. 7 is a diagram illustrating an example of frequency conversion in an existing satellite receiver.

  FIG. 1 is a diagram illustrating an example of a satellite communication system using VSAT. This system is formed with a communication satellite SAT on a geostationary orbit carrying a plurality of transponders as a core. On the ground side, fixed stations 11, 14 to 1n are installed, for example, at the prefectural office location. The in-vehicle station 12 or the portable station 13 can be installed at a disaster site or the like. The fixed stations 11, 14 to 1n, the in-vehicle station 12, and the portable station 13 are each provided with a VSAT station, and can communicate with each other via the communication satellite SAT.

  For example, the image of the disaster site can be transmitted to a main site (fixed station 11) or each site (fixed stations 14 to 1n) via a satellite line to help grasp the disaster situation. It can also be used for information sharing and disaster response discussions between related departments through VoIP (Voice over IP) calls and TV conferences over satellite channels. This type of system is often built as one of the local government disaster prevention systems.

  For example, there is an artificial satellite equipped with 23 transponders. Of these, a local government disaster prevention system using three transponders adjacent to each other in the band is known. In this type of system, when a call origination request is generated from a certain station, a shared line reserved in advance for each earth station (fixed station 11, 14-1n, in-vehicle station 12, and portable station 13) is selected. A line control device (not shown) selects a connectable line and assigns it to a call originator. Such a line assignment method is called DAMA (Demand Assignment Multiple Access).

  In the embodiment, a Ku-band artificial satellite is assumed, and the uplink band from the ground to the satellite is set to 14 GHz to 14.5 GHz, and the downlink band from the satellite to the ground is set to 12.25 GHz to 12.75 GHz. Taking so-called multi-channel VSAT as an example, using three transponders provided in an artificial satellite, the simultaneous use band is 120 MHz (38 MHz × 3 transponders + guard band), and the variable range is a maximum of 500 MHz. Assume that transmission / reception is performed by a carrier.

  The cover band of each transponder is denoted as T1, T2, T3. That is, the bandwidths of T1, T2, and T3 are each 38 MHz, and there are 2 MHz guard bands between T1 and T2 and between T2 and T3. In other words, each of the cover bands of the three transponders has a bandwidth range of 38 MHz, which is arranged with a guard band of 2 MHz in the Ku band.

FIG. 2 is a functional block diagram illustrating an example of a VSAT station according to the embodiment. In the VSAT station 1 shown in FIG. 2, the transmission signal is QPSK (Quadrature Phase Shift Keying) encoded by the encoding unit 10 and then input to the transmission IF (Intermediate Frequency) unit 20. The transmission IF unit 20 performs quadrature digital modulation of the carrier wave based on the QPSK-encoded digital data and inputs the result to a transmission RF (Radio Frequency) unit 40. The transmission RF unit 40 up-converts the quadrature digital modulated signal (analog) to the transmission band, and transmits the signal to the communication satellite SAT via the transmission antenna 51.
The antenna element of the transmission antenna 51 may be formed on the planar antenna ANT. Further, the antenna element of the receiving antenna 52 may be formed on the same planar antenna ANT.

  On the other hand, a radio signal arriving from the transponder of the communication satellite SAT is captured by the reception antenna 52 and sent to the reception RF unit 60. The reception RF unit 60 down-converts the radio signal, converts it to a reception IF signal, and inputs it to the reception IF unit 70.

  On the other hand, the local oscillator 30 generates a local frequency signal. The frequency Lo of the local frequency signal is a frequency included in the guard band between T1 and T2, or a frequency included in the guard band between T2 and T3. In the embodiment, the frequency Lo of the local frequency signal is the center (center) frequency between T1 and T2.

  The reception IF unit 70 mixes the local frequency signal and the reception IF signal and inputs them to the demodulation unit 80. The demodulator 80 orthogonally demodulates the received IF signal to generate baseband IQ complex signals that are orthogonal to each other, and inputs them to the processing circuit 90. The processing circuit 90 reproduces the transmission data included in the radio signal received from the transponder from the IQ complex signal, and obtains a received signal.

  In FIG. 2, at least the encoding unit 10 and the demodulating unit 80 can be formed on a common FPGA (Field Programmable Gate Array). As a result, speeding up and downsizing of processing can be promoted. Further, by reducing the band of the analog / digital converter provided in the reception IF unit, it is possible to promote a significant reduction in size and cost. The novel method will be described in detail below.

  FIG. 3 is a functional block diagram illustrating an example of the reception IF unit 70 and the demodulation unit 80. The reception IF signal is input to the quadrature demodulator 71 of the reception IF unit 70, mixed with the local frequency signal of the local oscillator 30, and quadrature demodulated to generate an I signal and a Q signal (IQ complex signal). For example, the I signal is input to the low pass filter (LPF) 72, and the Q signal is input to the low pass filter (LPF) 73. The LPFs 72 and 73 function as channel filters (roll-off filters), respectively, and extract desired wave components from the I signal and Q signal, respectively. This desired wave component is input to the analog / digital converter 74.

  The analog / digital converter 74 performs analog / digital conversion on the desired wave component to extract encoded data. The encoded data is input to the demodulator 80, for example, to four orthogonal decoders 811 to 814. The orthogonal decoders 811 to 814 decode the encoded data, and the decoders 821 to 824 reproduce the transmission data for each channel.

  Next, the operation of the above configuration will be described. As described above, it is assumed that a simultaneous use band is 120 MHz and a variable range is transmitted and received by a plurality of carriers in a band with a maximum of 500 MHz. Further, the transmission uplink band was set to 14 GHz to 14.5 GHz, and the reception downlink band was set to 12.25 GHz to 12.75 GHz. Furthermore, it is considered to provide a connection port for video IRD in the receiving system. Then, assuming that a general-purpose receiving module is used, the band of the reception IF signal is 950 MHz to 1450 MHz. Further, the local frequency for down-conversion in the reception RF unit 60 is determined to be 11.3 GHz, and the transmission frequency in the encoding unit 10 is determined to be 2700 MHz to 3200 MHz.

  FIG. 4 is a diagram illustrating an example of the relationship between the cover bands T1, T2, and T3 of each transponder and the frequency Lo of the local frequency signal. In the embodiment, the frequency Lo of the local frequency signal is set at the center between T1 and T2.

  FIG. 5 is a diagram for explaining direct conversion based on the frequency setting of FIG. That is, by setting Lo at the center of T1 and T2, the reception IF signal converted from 950 MHz to 1450 MHz by the reception RF unit 60 is shown in FIG. 5B from the state shown in FIG. Thus, T1 and T2 are down-converted to baseband in a state where they overlap and overlap each other. That is, an intermediate frequency signal of 950 MHz to 1450 MHz is dropped to a BB (baseband) signal of −40 to +80 MHz. In the demodulator 80, a desired signal (for example, BW = 50 kHz to 8 MHz) is selected from the IQ complex signal of −40 to +80 MHz, and only the desired signal is extracted and demodulated by the channel filter (roll-off filter).

  That is, by performing quadrature demodulation using the local frequency Lo set as described above, the signal band input to the analog / digital converter 74 is reduced from the band of 120 MHz to the equivalent of 80 MHz as shown in FIG. Will be.

  FIG. 6 schematically shows the relationship between the cover bands T1, T2, T3 and the guard band (GB) after down-conversion. Here, bands T1 and T2 close to DC (direct current) are overlapped. In the IQ complex signal, each signal in the overlapped region can be easily identified. For example, in the processing circuit 90 (FIG. 2), both signals can be identified based on the rotation direction of the complex vector in the IQ space.

  As shown in FIG. 7, in the existing satellite receiver, the received signal (FIG. 7A) once frequency-converted into an IF signal is baseband converted with the local frequency Lo at a position not affected by the DC offset (FIG. 7). 7 (b)), analog / digital conversion is performed from this state. Therefore, since the band corresponding to 120 MHz must be sampled as it is, the circuit scale becomes large. For example, since a sampling frequency of 260 MHz or more is required, it is difficult to use a general-purpose product. Such an ultra-high speed analog / digital converter is expensive and difficult to incorporate in an FPGA. Further, since the attenuation characteristic with respect to the frequency of the IF signal is severe, the image removal filter (analog IF filter) is also required to have severe characteristics. Under such circumstances, the expansion of the circuit scale has been spurred, which is disadvantageous in terms of cost.

  Even if an attempt is made to reduce the baseband band by dropping it directly to the BB (baseband) at the center frequency of the coverband T2 of the second transponder, the influence of the DC offset will be significant. In the case of multichannel, since the level of one carrier is very low, it tends to be greatly affected by the DC offset generated by direct demodulation.

  Therefore, in the embodiment, baseband conversion is performed on the signals in the T1, T2, and T3 bands converted into IF signals at the local frequency Lo set in the guard band between T1 and T2. That is, the center of the 2 MHz guard band (GB) provided between the transponders is down-converted to 0 Hz. As a result, the influence of the DC offset does not occur in the multi-channel and broadband VSAT, and the signal band input to the analog / digital converter can be reduced.

Therefore, the sampling frequency required for the analog / digital converter can be suppressed to about 200 MHz, the circuit scale can be reduced, and adverse effects such as deterioration of the reception S / N ratio due to DC offset can be avoided. Furthermore, the influence of the DC offset can be eliminated by suppressing the DC component outside the band by the channel filter.
From these facts, according to the embodiment, it is possible to provide a satellite receiver and a receiving method thereof that are simple in structure and light in weight.

The present invention is not limited to the above embodiment. For example, in FIG. 4, the frequency Lo of the local frequency signal is set at the center between T1 and T2. Alternatively, the frequency Lo of the local frequency signal can be set at the center between T2 and T3.
In FIG. 2, antennas (transmission antenna 51 and reception antenna 52) are separately provided in the transmission system and the reception system. Instead of this, a common antenna may be used in the transmission system and the reception system by using a hybrid circuit, an ODM, or the like.

  Although an embodiment of the present invention has been described, this embodiment is presented as an example and is not intended to limit the scope of the invention. The novel embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. This embodiment and its modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... VSAT station, 10 ... Encoding part, 11 ... Fixed station, 12 ... In-vehicle station, 13 ... Portable station, 14-1n ... Fixed station, 20 ... Transmission IF part, 30 ... Local oscillator, 40 ... Transmission RF part , 51 ... transmitting antenna, 52 ... receiving antenna, 60 ... receiving RF section, 70 ... receiving IF section, 71 ... quadrature demodulator, 72, 73 ... low-pass filter, 74 ... analog / digital converter, 80 ... demodulating section, 90 ... Processing circuit, 811 to 814 ... Orthogonal decoder, 821 to 824 ... Decoder, T1, T2 ... Coverband

Claims (6)

A high-frequency unit that converts a radio signal received from an artificial satellite including a plurality of transponders into an intermediate frequency signal;
A local oscillator that outputs a local frequency signal of a frequency included in a guard band between cover bands of each of the plurality of transponders;
A quadrature demodulator that orthogonally demodulates the intermediate frequency signal using a local frequency signal output from the local oscillator and generates orthogonal baseband IQ complex signals;
A satellite receiver comprising: a signal processing unit that reproduces transmission data included in the radio signal from the IQ complex signal. The signal processing unit
A filter for extracting a desired wave component from the IQ complex signal;
An analog / digital converter for extracting encoded data by analog / digital conversion of the desired wave component;
The satellite receiver according to claim 1, further comprising: a decoding unit that decodes the encoded data and reproduces the transmission data.   2. The satellite receiver according to claim 1, wherein each cover band of the plurality of transponders has a bandwidth range of 38 MHz arranged in a Ku band with a guard band of 2 MHz. Converts radio signals received from satellites equipped with multiple transponders into intermediate frequency signals,
Orthogonally demodulating the intermediate frequency signal using a local frequency signal of a frequency included in a guard band between the cover bands of each of the plurality of transponders to generate orthogonal baseband IQ complex signals;
A reception method of reproducing transmission data included in the radio signal from the IQ complex signal. Said playing is
Extracting a desired wave component from the IQ complex signal;
Analog / digital conversion of the desired wave component to extract encoded data;
The reception method according to claim 4, further comprising: decoding the encoded data to reproduce the transmission data.   5. The reception method according to claim 4, wherein a cover band of each of the plurality of transponders has a bandwidth range of 38 MHz, which is arranged with a guard band of 2 MHz in the Ku band.